U.S. patent application number 13/451621 was filed with the patent office on 2012-08-09 for methods and systems for chemical composition measurement and monitoring using a rotating filter spectrometer.
This patent application is currently assigned to Pason Systems Corp.. Invention is credited to David Bonyuet, Vidi A. Saptari.
Application Number | 20120200855 13/451621 |
Document ID | / |
Family ID | 40849227 |
Filed Date | 2012-08-09 |
United States Patent
Application |
20120200855 |
Kind Code |
A1 |
Bonyuet; David ; et
al. |
August 9, 2012 |
Methods and Systems for Chemical Composition Measurement and
Monitoring Using a Rotating Filter Spectrometer
Abstract
The invention relates to methods and systems for measuring
and/or monitoring the chemical composition of a sample (e.g., a
process stream), and/or detecting specific substances or compounds
in a sample, using light spectroscopy such as absorption, emission
and fluorescence spectroscopy. In certain embodiments, the
invention relates to spectrometers with rotating narrow-band
interference optical filter(s) to measure light intensity as a
function of wavelength. More specifically, in certain embodiments,
the invention relates to a spectrometer system with a rotatable
filter assembly with a position detector rigidly attached thereto,
and, in certain embodiments, the further use of various
oversampling methods and techniques described herein, made
particularly useful in conjunction with the rotatable filter
assembly. In preferred embodiments, the rotatable filter is tilted
with respect to the rotation axis, thereby providing surprisingly
improved measurement stability and significantly improved control
of the wavelength coverage of the filter spectrometer.
Inventors: |
Bonyuet; David; (Watertown,
MA) ; Saptari; Vidi A.; (Cambridge, MA) |
Assignee: |
Pason Systems Corp.
Calgary Alberta
CA
|
Family ID: |
40849227 |
Appl. No.: |
13/451621 |
Filed: |
April 20, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12427485 |
Apr 21, 2009 |
8184293 |
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13451621 |
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61084985 |
Jul 30, 2008 |
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Current U.S.
Class: |
356/418 |
Current CPC
Class: |
G01J 3/32 20130101; G01J
3/42 20130101; G01N 21/33 20130101; G01J 3/06 20130101; G01J 3/0235
20130101; G01N 21/314 20130101; G01J 2003/1247 20130101; G01N
21/3504 20130101; G01N 21/274 20130101; G01N 2021/3174 20130101;
G01N 2201/129 20130101; G01J 3/02 20130101 |
Class at
Publication: |
356/418 |
International
Class: |
G01N 21/27 20060101
G01N021/27 |
Claims
1-3. (canceled)
4. A spectroscopic system for detecting electromagnetic radiation
that has passed through or is reflected from a sample, the system
comprising: an electromagnetic radiation source; a rotatable filter
assembly configured to filter a beam of electromagnetic radiation
produced by the electromagnetic radiation source; a motor coupled
to the rotatable filter assembly; a position detector comprising at
least one component rigidly attached to the rotatable filter
assembly, the position detector configured to detect an angular
position of the rotatable filter assembly; and an electromagnetic
radiation detector configured to detect electromagnetic radiation
that has passed through or is reflected from a sample.
5-91. (canceled)
Description
RELATED APPLICATION
[0001] This Application claims benefit of U.S. Provisional Patent
Application No. 61/084,985 filed on Jul. 30, 2008, the text of
which is incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
[0002] The invention relates generally to spectroscopic methods and
systems. More particularly, in certain embodiments, the invention
relates to methods and systems for measuring and/or monitoring the
chemical composition of a sample (e.g., a process stream), and/or
detecting substances or compounds in a sample, using light
spectroscopy.
BACKGROUND OF THE INVENTION
[0003] Several chemical composition measurement devices using light
spectrometers are currently commercially available. Examples of the
types of spectrometers currently used include Fourier transform
infrared spectrometer (FTIR), dispersive spectrometer (spectrograph
or monochromator) and linear variable filter (LVF) spectrometer.
FTIR based devices use Michelson interferometers and have generally
been considered to provide the highest performance, due to their
high optical throughput, which enables high-sensitivity
measurements. In contrast, dispersive and linear variable filter
spectrometers have significantly lower optical throughput and thus
lower sensitivity performance. However, dispersive and linear
variable filter spectrometers generally provide simpler and more
rugged instrumentation, and are less expensive to manufacture.
[0004] Another type of chemical composition measurement and
monitoring device that is widely used, in particular, in the field
of gas monitoring, is non-dispersive infrared (NDIR) devices. These
devices use fixed narrowband optical filters to select a particular
wavelength band region. They have high optical throughput, rivaling
that of FTIR based devices, and thus provide high-sensitivity
measurement. This type of device, however, is generally not
considered to be a spectrometer, as it does not measure light
intensity as a function of wavelength; rather, it provides a single
measurement value corresponding to the light intensity at a
particular wavelength band. For this reason, each device (employing
one filter, one photo-detector and one light source) can only
measure one compound. Therefore, such devices are not considered to
be chemical "composition" measuring devices.
[0005] The transmitted wavelength band of a narrowband optical
filter, such as that used in NDIR instruments, can be varied or
"tuned" by varying the angle of incidence (U.S. Pat. No. 4,040,747
to Webster, 1977 and U.S. Pat. No. 2,834,246 to Foskett, 1958, both
of which are incorporated herein by reference). Such methods enable
the measurement of optical signals from multiple wavelengths or
wavelength bands using only a single optical filter, light source
and detector, thus potentially creating a simple, low-cost,
high-throughput spectrometer.
[0006] One method of varying the incident angle is to continuously
rotate the filter in one direction and capture the data at the
relevant angular positions. This type of continuously-rotating
filter spectrometers has been described (U.S. Pat. No. 4,040,747 to
Webster, 1977, U.S. Pat. No. 2,834,246 to Foskett, 1958, U.S. Pat
No. 5,268,745 to Goody, 1993, U.S. Pat No. 7,099,003 to Saptari,
2006, all of which are incorporated herein by reference). However,
these devices have not been in significant commercial use. FTIR
spectrometers, grating based spectrometers and LVF spectrometers
are still by far the most commonly used hardware for chemical
composition monitoring, despite the potential advantages for the
rotating tunable filter instruments.
[0007] There are weaknesses of the previous rotating tunable filter
systems which prevent them from being used in a commercial setting
as chemical composition measuring or monitoring devices. For
example, these systems lack measurement stability and robustness
due to wavelength instability, spectral interferences,
environmental variations and/or instrumental changes. Such systems
also lack versatility, in particular, in that they are not able to
provide wide spectral coverage. Furthermore, there are difficulties
in volume manufacturing, in particular, difficulties in producing
reproducible instruments that are interchangeable without each
instrument requiring empirical sample based calibration.
SUMMARY OF THE INVENTION
[0008] The invention provides methods and systems for measuring
and/or monitoring the chemical composition of a sample (e.g., a
process stream in an industrial setting), using a spectrometer with
rotating narrow-band interference optical filter(s). In preferred
embodiments, the spectrometer system features a rotatable filter
assembly with a position detector rigidly attached thereto,
providing more accurate and robust detection. The rotatable filter
is preferably tilted with respect to the rotation axis, thereby
providing surprisingly improved measurement stability and
significantly improved control of the wavelength coverage of the
filter spectrometer. Also, in certain embodiments, the invention
includes methods of using such spectrometers for measuring and
monitoring chemical composition of compounds in gas, liquid and/or
solid forms, for example, in both laboratory and non-lab (e.g.,
industrial) settings.
[0009] Various oversampling methods and techniques are also
presented herein, which are found to be particularly useful when
employed in conjunction with a spectrometer with the rotatable
filter assembly feature as described herein. In certain
embodiments, the invention includes methods of using such
spectrometers for measuring and monitoring chemical composition of
compounds in gas, liquid and/or solid forms, for example, in both
laboratory and non-lab (e.g., industrial) settings.
[0010] In certain embodiments, the invention provides a rotating
filter spectrometer for chemical composition measurement and
monitoring, employs one or multiple light sources, one or multiple
photo-detectors, one or multiple narrow-band optical interference
filters, a DC motor, a position encoder, an analog-to-digital
conversion device, and a computing unit. In preferred embodiments,
the narrow-band optical filter(s) are rigidly mounted on a rotating
mechanical assembly driven by a DC motor. The rotating filter
assembly is positioned relative to a collimated light beam from the
light source such that the axis of rotation is perpendicular to the
light beam or, preferably, positioned such that the axis of
rotation is slightly non-perpendicular to the light beam, such
non-perpendicular conformation resulting in surprisingly improved
measurement stability due to apparent suppression of back-reflected
or stray light, and resulting in significantly improved control of
the wavelength coverage of the filter spectrometer, given the
filter characteristics and the angular coverage of the mechanical
system.
[0011] In preferred embodiments, the rotating filter assembly
rotates continuously in one angular direction. A rotary positional
encoder is rigidly attached to the rotating filter assembly such
that there is no relative displacement or mechanical "compliance"
or "play" between it and the rotating filter assembly. The digital
pulses generated by the encoder during motion are used to clock the
analog-to-digital conversion of the signal collected by the
photo-detector. Furthermore, the encoder and its processing
electronics are designed, configured and/or selected in such a way
that it produces significantly more pulses-per-rotation than what
is required to accurately measure the relevant spectral features.
The spectral signal is over-sampled. A convolution algorithm is
then preferably applied to digitally process the recorded spectrum
to enhance wavelength stability or repeatability and to improve
spectral signal-to-noise ratio.
[0012] In certain embodiments, inherent or deliberately-introduced
spectral features are used to lock the relative position of the
encoder with respect to the rotating filter assembly. The spectral
features may be those due to the spectral characteristics of the
light source, system's optical components, and/or the sample
compound itself. Such methods ensure wavelength stability despite
alignment changes due to mechanical forces or temperature
changes.
[0013] A variable gain amplifier is preferably employed to
automatically adjust the photo-detector signal amplification gain
in real-time. The gain profile may be scheduled based upon the
location of the rotating filter assembly, or updated automatically
based upon the magnitude of the received signal. Such a feature
enables measurement of distinctly different spectral regions, such
as measurement at the near infrared and the mid infrared regions
simultaneously, without saturating the analog-to-digital circuitry.
Similarly, the light source intensity may be varied to further
optimize the measurement dynamic range and to better observe weak
spectral features.
[0014] In certain embodiments, multiple regression regions and
calibration matrices, combined with cross-analysis, are used to
enhance robustness and accuracy of multi-compound measurement as
well as measurement in highly complex sample matrices. Each
calibration matrix can be optimized for a particular target
compound or features of the target compounds. The effects of
nonlinearities can be significantly suppressed.
[0015] In certain embodiments, an adaptive regression analysis is
employed to account for spectral baseline variations that may have
complex shapes due to the filter's non-linear wavelength-angle
function. The algorithm automatically and continually updates to
compensate for the baseline variations, as well as other spectral
variations such as those due to light interactions with unknown,
interfering compounds.
[0016] A differential measurement may be employed in applications
monitoring certain processes or reactions, for example, where the
input and output streams are available for analysis. The method
suppresses the effects of instrumental and environmental changes,
as well as minimizes the effects of sample background
interferences.
[0017] Embodiments of the invention provide methods, systems
(including apparatus) for chemical composition measurement and
monitoring in gas, liquid and/or solid samples which utilize a
single or multiple continuously rotating narrow-band filters. In
certain embodiments, the invention provides negligible wavelength
instability or drift due to various environmental disturbances such
as vibrations and temperature variations over a long period of
time. Embodiments of the invention also provide wide spectral or
wavelength coverage with optimum use of the measurement dynamic
range throughout the analysis range, suitable for simultaneous
measurement and/or monitoring of multiple compounds. The systems
effectively compensate for spectral baseline instability and can be
built and manufactured consistently (without requiring extensive,
individual-machine calibration) and relatively inexpensively.
[0018] The systems and methods can be used for continuous
monitoring of gas, liquid, and/or solid chemical composition (%
levels), for example, for monitoring production throughput and
quality, e.g., in process streams. They can also be used for gas,
liquid, or solid phase trace species monitoring (ppm or ppb
levels), for example, impurity detection and monitoring, e.g., in
process streams. Embodiments may also provide ambient monitoring
for safety purposes. The systems and methods described herein may
be applied, for example, in the petrochemical, bioreactor
(biofuel), pharmaceutical, food and beverage, specialty chemical,
and/or alternative energy industries.
[0019] For example, an embodiment of the invention provides
combustion process monitoring (e.g., alternative energy production
using a bioreactor) for the monitoring of any one or more of the
following process gases: CO, CO.sub.2, O.sub.2, CH.sub.4 (methane),
N.sub.2O (nitrous oxide). In other embodiments, the invention
provides systems and/or methods for monitoring trace levels (e.g.,
ppm or sub-ppm) of sulfur compounds (e.g., dimethyl sulfide,
dimethyl disulfide, carbonyl sulfide, hydrogen sulfide, etc.) in a
natural gas line, for example, in a fuel cell-based power plant. In
yet another embodiment, the invention provides a system and/or
method for monitoring trace levels (e.g., ppm or sub-ppm) of CO,
CO.sub.2, H.sub.2O (moisture), THC (total hydrocarbon) gases in
N.sub.2 or He, for example, for specialty chemical manufacturers.
Other example applications of the methods and systems of the
invention include the monitoring of trace water in fuels, the
monitoring of aqueous alcohols, and the monitoring of glucose,
lactate, ammonia, and/or glutamine during fermentation
processes.
[0020] In one aspect, the invention provides a spectroscopic system
for detecting electromagnetic radiation that has passed through or
is reflected from a sample, the system including an electromagnetic
radiation source and a rotatable filter assembly configured to
filter a beam of electromagnetic radiation produced by the
electromagnetic radiation source, where the assembly includes one
or more bandpass optical interference filters, and where the
rotatable filter assembly is configured to rotate to provide
continuous adjustment of the incident angle of the electromagnetic
beam onto the one or more optical interference filters, thereby
providing a continuous wavelength sweep in a single wavelength band
or multiple wavelength bands. One or more of the bandpass filters
is configured such that the surface of the filter is not exactly
perpendicular to the electromagnetic beam at any point during the
continuous adjustment (e.g., the surface is displaced from
perpendicular by up to about 3 degrees, by up to about 5 degrees,
by up to about 10 degrees, by up to about 20 degrees, or by up to
about 30 degrees). The system also includes a motor coupled to the
rotatable filter assembly and an electromagnetic radiation detector
configured to detect electromagnetic radiation that has passed
through or is reflected from the sample. In certain embodiments,
the rotatable filter assembly includes a narrow-band interference
filter or plurality of narrow-band interference filters. In certain
embodiments, the rotatable filter assembly includes an edge
interference filter or plurality of edge interference filters (such
as low-pass or high-pass interference filters).
[0021] The description of elements of the embodiments of other
aspects of the invention can be applied to this aspect of the
invention as well.
[0022] In another aspect, the invention provides a spectroscopic
system for detecting electromagnetic radiation that has passed
through or is reflected from a sample including an electromagnetic
radiation source; a rotatable filter assembly configured to filter
a beam of electromagnetic radiation produced by the electromagnetic
radiation source; a motor coupled to the rotatable filter assembly;
a position detector including at least one component rigidly
attached to the rotatable filter assembly, the position detector is
configured to detect an angular position of the rotatable filter
assembly; and an electromagnetic radiation detector configured to
detect electromagnetic radiation that has passed through or is
reflected from a sample.
[0023] In certain embodiments, the rotatable filter assembly is
configured to rotate about an axis substantially perpendicular to a
path of a beam of electromagnetic radiation produced by the
electromagnetic radiation source. In certain embodiments, the
rotatable filter assembly is configured to rotate about an axis
non-perpendicular to a path of a beam of electromagnetic radiation
produced by the electromagnetic radiation source at an angle within
a range from about 60 degrees to less than 90 degrees (e.g., 89.99
degrees). In certain embodiments, the rotatable filter assembly
includes a narrow-band interference filter.
[0024] In certain embodiments, the rotatable filter assembly
includes a plurality of filters. In certain embodiments, the
rotatable filter assembly includes at least three filters.
[0025] In certain embodiments, the surface of the filter(s) is
parallel to the axis of rotation of the rotatable filter assembly.
In certain embodiments, the filter(s) is angularly tilted about an
axis perpendicular to the axis of rotation of the rotatable filter
assembly and the axis normal to the surface of the filter.
[0026] In certain embodiments, the spectroscopic system includes a
controller configured to adjust a rotational velocity of the
rotatable filter assembly. In certain embodiments, the position
detector includes an encoder configured to produce at least a first
signal including a series of digital pulses at a first frequency,
each digital pulse corresponding to an angular position of the
rotatable filter assembly. In certain embodiments, the first
frequency is a clock frequency. In certain embodiments, the encoder
is configured to produce a second signal, and the spectroscopic
system includes an encoder signal processing module configured to
combine the first and second signals into a third signal. In
certain embodiments, the third signal includes a series of digital
pulses having at least double the first frequency. In certain
embodiments, the encoder includes an edge detector configured to
detect an edge of each of at least two signals produced by the
encoder and to thereby produce a signal including a series of
digital pulses having at least quadruple the first frequency.
[0027] In certain embodiments, the encoder is rigidly attached to
the rotatable filter assembly. In certain embodiments, the system
includes a speed-reduction mechanism configured to control a
velocity of the rotatable filter assembly. In certain embodiments,
the speed-reduction mechanism is configured to control the velocity
using a digital feedback control.
[0028] In certain embodiments, the encoder is configured to produce
significantly more digital pulses per rotation of the rotatable
filter assembly than are necessary to accurately reproduce an
analog signal from the electromagnetic radiation detector. In
certain embodiments, the encoder is configured to digitize the
analog signal at a frequency greater than a Nyquist criterion
corresponding to the analog signal. In certain embodiments, the
encoder is configured to digitize the analog signal at a frequency
greater than 5 times the Nyquist criterion. In certain embodiments,
the encoder is configured to digitize the analog signal at a
frequency at least 8 times the Nyquist criterion. In certain
embodiments, the encoder is configured to digitize the analog
signal at a frequency at least 10 times the Nyquist criterion. In
certain embodiments, the encoder is configured to digitize the
analog signal with at least 1000 pulses per rotation of the
rotatable filter assembly.
[0029] In certain embodiments, the spectroscopic system includes a
variable gain amplifier configured to convert a light signal from
the electromagnetic radiation detector into an electrical signal.
In certain embodiments, the variable gain amplifier is in
communication with the position detector and is configured to
automatically adjust a gain profile of a signal received from the
electromagnetic radiation detector based on a detected angular
position of the rotatable filter assembly. In certain embodiments,
the amplifier is configured to automatically adjust a gain profile
of a signal received from the electromagnetic radiation detector
based on a magnitude of the signal.
[0030] In certain embodiments, the spectroscopic system includes a
processor configured to apply a convolution function to a spectral
signal from the electromagnetic radiation detector, thereby
enhancing wavelength stability and/or repeatability, and/or thereby
improving signal-to-noise ratio. In certain embodiments, a width of
the convolution function is as great as possible without altering
or broadening spectral features of the spectral signal.
[0031] In certain embodiments, the spectroscopic system includes a
processor configured to apply a baseline correction algorithm to a
spectral signal from the electromagnetic radiation detector,
thereby enhancing long-term measurement stability.
[0032] In certain embodiments, the spectroscopic system includes a
plurality of electromagnetic radiation sources, thereby enabling
detection of electromagnetic radiation over a broader spectrum
and/or over multiple spectra. In certain embodiments, the plurality
of electromagnetic radiation sources includes a UV radiation source
and an IR radiation source. In certain embodiments, the
spectroscopic system includes an analog-to-digital acquisition
mechanism in communication with the electromagnetic radiation
detector and the position detector, where the analog-to-digital
acquisition mechanism is configured to digitize, store, and/or
process data corresponding to the detected electromagnetic
radiation. The spectroscopic system may include a computer or may
otherwise share input and output with a computer 2802 (e.g., a
computer internal or external to the spectroscopic system), the
computer including software for digitizing, receiving, storing, and
or processing data corresponding to the detected electromagnetic
radiation and/or signals created by such detected electromagnetic
radiation as illustrated in FIG. 28. The computer may also include
a keyboard or other portal for user input, and a screen for display
of data to the user. The computer may include software for process
control, data acquisition, data processing, and/or output
representation. The spectroscopic system may include a wireless
system for acquisition of data and/or system control. For example,
the wireless system may allow wireless data transfer from and/or to
a computer, allowing wireless input and/or output (and/or system
control) by/to a user via a user interface connected to the
computer, such as a keyboard and/or display screen. The
spectroscopic system may also include a battery system configured
to enable stand-alone operation capability.
[0033] The description of elements of the embodiments of other
aspects of the invention can be applied to this aspect of the
invention as well.
[0034] In another aspect, the invention provides a spectroscopic
system for detecting electromagnetic radiation that has passed
through or is reflected from a sample, including an electromagnetic
radiation source; a rotatable filter assembly configured to filter
a beam of electromagnetic radiation produced by the electromagnetic
radiation source; a motor coupled to the rotatable filter assembly,
an electromagnetic radiation detector configured to detect
electromagnetic radiation that has passed through or is reflected
from a sample and to output a corresponding analog spectral signal;
and a position detector configured to detect an angular position of
the rotatable filter assembly, the position detector including an
encoder configured to produce at least a first signal including a
series of digital pulses at a first frequency, each digital pulse
corresponding to an angular position of the rotatable filter
assembly, wherein the encoder is configured to produce
significantly more digital pulses per rotation of the rotatable
filter assembly than are necessary to reproduce the analog spectral
signal.
[0035] In certain embodiments, the encoder is configured to
digitize the analog signal at a frequency greater than a Nyquist
criterion corresponding to the analog signal. In certain
embodiments, the encoder is configured to digitize the analog
signal at a frequency greater than 5 times the Nyquist criterion.
In certain embodiments, the encoder is configured to digitize the
analog signal at a frequency at least 8 times the Nyquist
criterion. In certain embodiments, the encoder is configured to
digitize the analog signal at a frequency at least 10 times the
Nyquist criterion. In certain embodiments, the encoder is
configured to digitize the analog signal with at least 1000 pulses
per rotation of the rotatable filter assembly.
[0036] The description of elements of the embodiments of other
aspects of the invention can be applied to this aspect of the
invention as well.
[0037] In another aspect, the invention provides a spectroscopic
system for detecting electromagnetic radiation that has passed
through or is reflected from a sample, including an electromagnetic
radiation source having a variable intensity; a filter assembly
configured to filter a beam of electromagnetic radiation produced
by the electromagnetic radiation source; a position detector
configured to detect a position of the filter assembly; a
controller configured to adjust the intensity of the
electromagnetic radiation source; and an electromagnetic radiation
detector configured to detect electromagnetic radiation that has
passed through or is reflected from a sample.
[0038] In certain embodiments, the filter assembly is rotatable and
the position detector is configured to detect an angular position
of the filter assembly.
[0039] In certain embodiments, the controller is in communication
with the position detector and is configured to adjust the
intensity of the electromagnetic radiation source based on a
detected position of the filter assembly. In certain embodiments,
the filter assembly includes a filter having an active portion and
an inactive portion and the controller is configured to decrease
the intensity of the electromagnetic radiation source when a beam
of electromagnetic radiation from the electromagnetic radiation
source is incident on an inactive portion of the filter.
[0040] In certain embodiments, the controller includes a voltage
regulator for controlling a voltage supplied to the electromagnetic
radiation source.
[0041] In certain embodiments, the spectroscopic system includes a
plurality of electromagnetic radiation sources. In certain
embodiments, the spectroscopic system includes a plurality of
electromagnetic radiation detectors.
[0042] In certain embodiments, the spectroscopic system includes a
variable gain amplifier configured to convert a light signal from
the electromagnetic radiation detector into an electrical signal.
In certain embodiments, the amplifier is in communication with the
position detector and is configured to automatically adjust a gain
profile of the electrical signal based on a detected position of
the filter assembly.
[0043] In certain embodiments, the filter assembly is configured
for rotation about an axis substantially perpendicular to a path of
a beam of electromagnetic radiation produced by the electromagnetic
radiation source.
[0044] The description of elements of the embodiments of other
aspects of the invention can be applied to this aspect of the
invention as well.
[0045] In another aspect, the invention provides a spectroscopic
system for monitoring electromagnetic radiation that has passed
through or is reflected from a sample including an electromagnetic
radiation source; a filter assembly configured to filter a beam of
electromagnetic radiation produced by the electromagnetic radiation
source; an electromagnetic radiation detector configured to detect
electromagnetic radiation that has passed through or is reflected
from a sample; and a processor in communication with the
electromagnetic radiation detector, the processor configured to:
(i) apply a first calibration spectrum to a first recorded spectrum
obtained from the electromagnetic radiation detector, thereby
determining a measure of one or more compounds in the sample; and
(ii) modify the first calibration spectrum to account for a
baseline variation of recorded spectra over time using at least a
second, subsequent recorded spectrum obtained from the
electromagnetic radiation detector.
[0046] The description of elements of the embodiments of other
aspects of the invention can be applied to this aspect of the
invention as well.
[0047] In another aspect, the invention provides a system for
monitoring a process including an electromagnetic radiation source;
a filter assembly configured to filter a beam of electromagnetic
radiation produced by the electromagnetic radiation source; a
sampling mechanism configured to alternately direct a sample from a
first stream associated with the monitored process into a sampling
area and direct a sample from a second stream associated with the
monitored process into the sampling area; an electromagnetic
radiation source configured to direct an electromagnetic radiation
beam from the electromagnetic radiation source to the sampling
area; an electromagnetic radiation detector configured to detect
electromagnetic radiation that has passed through or is reflected
from the sampling area; and a processor configured to: (i) obtain a
first spectrum corresponding to the first stream; (ii) store the
first spectrum as a baseline spectrum; and (iii) obtain a second
spectrum from the second stream using the baseline spectrum,
wherein the second spectrum reflects a compositional difference
between the first and second streams.
[0048] In certain embodiments, the sampling mechanism includes a
solenoid valve for switching between the first and second streams.
In certain embodiments, the first stream is an input stream to the
monitored process and the second stream is an output stream from
the monitored process. In certain embodiments, the first stream is
an output stream from the monitored process and the second stream
is an input stream to the monitored process.
[0049] The description of elements of the embodiments of other
aspects of the invention can be applied to this aspect of the
invention as well.
[0050] In another aspect, the invention provides a spectroscopic
method for detecting electromagnetic radiation that has passed
through or is reflected from a sample, including: filtering a beam
from an electromagnetic radiation source with a rotating filter
assembly; detecting an angular position of the rotating filter
assembly with a position detector having at least one component
rigidly coupled to the rotating filter assembly; intercepting the
beam with a sample; detecting the beam with an electromagnetic
radiation detector; and processing a spectral data signal from the
electromagnetic radiation detector to produce chemical information
about the sample.
[0051] In certain embodiments, the rotating filter assembly is
configured to rotate about an axis substantially perpendicular to a
path of a beam of electromagnetic radiation produced by the
electromagnetic radiation source. In certain embodiments, the
rotating filter assembly includes a narrow-band interference
filter. In certain embodiments, the rotating filter assembly
includes at least three filters.
[0052] In certain embodiments, the position detector includes an
encoder configured to produce at least a first signal comprising a
series of digital pulses at a first frequency, each digital pulse
corresponding to an angular position of the rotating filter
assembly.
[0053] In certain embodiments, the method includes digitizing an
analog spectral signal from the electromagnetic radiation detector
is performed. In certain embodiments, digitizing is performed at a
frequency significantly greater than necessary to accurately
reproduce the analog spectral signal; digitizing is performed at a
frequency greater than a Nyquist criterion corresponding to the
analog spectral signal; and/or digitizing is performed at a
frequency greater than at least ten times the Nyquist
criterion.
[0054] In certain embodiments, a step of applying a convolution
function to a spectral signal from the electromagnetic radiation
detector is performed to enhance wavelength stability and/or
repeatability, and/or to improve signal-to-noise ratio.
[0055] The description of elements of the embodiments of other
aspects of the invention can be applied to this aspect of the
invention as well.
[0056] In yet another aspect, the invention provides a
spectroscopic method for detecting electromagnetic radiation that
has passed through or is reflected from a sample to produce
chemical information about the sample, the method including:
filtering a beam from an electromagnetic radiation source with a
rotating filter assembly; intercepting the beam with a sample;
detecting the beam with an electromagnetic radiation detector
configured to output an analog spectral signal; detecting an
angular position of the rotating filter assembly with a position
detector, the position detector comprising an encoder configured to
produce at least a first signal comprising a series of digital
pulses at a first frequency, each digital pulse corresponding to an
angular position of the rotating filter assembly, wherein the
encoder is configured to produce significantly more digital pulses
per rotation of the rotating filter assembly than are necessary to
reproduce the analog spectral signal; digitizing the analog
spectral signal using the first frequency as a clock frequency; and
processing the digitized analog spectral signal to produce chemical
information about the sample.
[0057] In certain embodiments, the first frequency is greater than
a Nyquist criterion corresponding to the analog spectral signal. In
certain embodiments, the first frequency corresponds to at least
1000 pulses per rotation of the rotating filter assembly (or,
alternatively, at least 2000, 1500, 1250, 900, 800, 700, 600, or
500 pulses per rotation).
[0058] The description of elements of the embodiments of other
aspects of the invention can be applied to this aspect of the
invention as well.
[0059] In yet another aspect, the invention provides a
spectroscopic method for detecting electromagnetic radiation that
has passed through or is reflected from a sample to produce
chemical information about the sample, including: filtering a beam
from an electromagnetic radiation source with a filter assembly,
the electromagnetic radiation source having a variable intensity;
intercepting the beam with a sample; detecting the beam with an
electromagnetic radiation detector; detecting a position of the
filter assembly with a position detector; adjusting the intensity
of the electromagnetic radiation source; and processing spectral
data from the electromagnetic radiation detector to produce
chemical information about the sample.
[0060] In certain embodiments, adjusting the intensity of the
electromagnetic radiation source is based on a detected position of
the filter assembly.
[0061] The description of elements of the embodiments of other
aspects of the invention can be applied to this aspect of the
invention as well.
[0062] In yet another aspect, the invention provides a
spectroscopic method for monitoring electromagnetic radiation that
has passed through or is reflected from a sample, the method
including: filtering a beam from an electromagnetic radiation
source with a filter assembly; intercepting the beam with a sample;
detecting the beam with an electromagnetic radiation detector;
applying a first calibration spectrum to a first recorded spectrum
obtained from the electromagnetic radiation detector, thereby
determining a measure of one or more compounds in the sample; and
modifying the first calibration spectrum to account for baseline
variation of the recorded spectra over time using at least a
second, subsequent recorded spectrum obtained from the
electromagnetic radiation detector.
[0063] The description of elements of the embodiments of other
aspects of the invention can be applied to this aspect of the
invention as well.
[0064] In yet another aspect, the invention provides a
spectroscopic method for monitoring a process, including: directing
a first sample from a first stream associated with the monitored
process into a sampling area; directing a second sample from a
second stream associated with the monitored process into the
sampling area; detecting filtered radiation that has passed through
or is reflected from the sampling area; determining a first
spectrum corresponding to the first stream; storing the first
spectrum as a baseline spectrum; and determining a second spectrum
from the second stream using the baseline spectrum, wherein the
second spectrum reflects a compositional difference between the
first and second streams.
[0065] In certain embodiments, the first stream is an input stream
to the monitored process and the second stream is an output stream
to the monitored process. In certain embodiments, the first stream
is an output stream to the monitored process and the second stream
is an input stream to the monitored process.
[0066] The description of elements of the embodiments of other
aspects of the invention can be applied to this aspect of the
invention as well.
[0067] In yet another aspect, the invention provides a method for
increasing the robustness and/or stability of the measurement,
including: obtaining a first spectrum from an electromagnetic
radiation detector; applying a classical least squares analysis to
the first spectrum using a principal calibration matrix to obtain
detection values; determining a residual magnitude by quantifying
how well the first spectrum fit the principal calibration matrix;
comparing the residual magnitude to a predetermined threshold to
determine if a threshold condition exists and, if a threshold
condition exists, creating a secondary reference matrix using the
first spectrum if a secondary reference matrix does not exist and,
if the secondary reference matrix exists, adding the first spectrum
to the secondary reference matrix as a row or a column; adding the
rows or columns of the secondary reference matrix to the principal
reference matrix to update the reference matrix; and reapplying a
classical least squares analysis to a second spectrum from an
electromagnetic radiation detector.
[0068] In certain embodiments, the size of the secondary reference
matrix is predetermined. In certain embodiments, determining a
residual magnitude comprises computing a mean of an absolute
function of a classical least squares fit of the first spectrum;
and/or determining a residual magnitude comprises computing a
maximum value of an absolute function of a classical least squares
fit of the first spectrum.
[0069] In certain embodiments, the threshold condition exists when
the residual magnitude exceeds a predetermined threshold value; the
threshold condition exists when the first spectrum is substantially
orthogonal to the principal calibration matrix; the reference
matrix comprises spectral data from a beam of electromagnetic
radiation that has not passed through a sample; the principal
calibration matrix comprises spectrum values corresponding only to
substances to be detected; and/or the principal calibration matrix
comprises spectrum values corresponding to substances to be
detected and other substances likely to be found together with the
substances to be detected.
[0070] The description of elements of the embodiments of other
aspects of the invention can be applied to this aspect of the
invention as well.
BRIEF DESCRIPTION OF DRAWINGS
[0071] The objects and features of the invention can be better
understood with reference to the drawings described below, and the
claims. The drawings are not necessarily to scale, emphasis instead
generally being placed upon illustrating the principles of the
invention. In the drawings, like numerals are used to indicate like
parts throughout the various views.
[0072] FIG. 1 is a block diagram of a spectroscopic system for
detecting radiation according to an illustrative embodiment of the
present invention.
[0073] FIG. 2 is a perspective view of a rotating filter assembly,
according to an illustrative embodiment of the invention.
[0074] FIG. 3 is schematic of an encoder signal processing circuit,
according to an illustrative embodiment of the invention.
[0075] FIG. 4 is an illustration of signal-to-noise ratio
improvement after application of a convolution function to a
spectral signal, according to an illustrative embodiment of the
invention.
[0076] FIG. 5 is a block diagram of a spectroscopic method
according to an illustrative embodiment of the present
invention.
[0077] FIG. 6 is a flow chart of a spectral processing method using
an adaptive algorithm according to an illustrative embodiment of
the present invention.
[0078] FIG. 7 is a flow chart for a spectral processing method
using an adaptive algorithm according to an illustrative embodiment
of the present invention.
[0079] FIG. 8 is an illustration of spectral data from an
experiment using a spectroscopic method according to an
illustrative embodiment of the present invention.
[0080] FIG. 9 is an illustration of spectral data from an
experiment using a spectroscopic method according to an
illustrative embodiment of the present invention.
[0081] FIG. 10 is an illustration of wavelength scale error caused
by wavelength shift.
[0082] FIG. 11 is a block diagram of a method for correcting for
wavelength scale error according to an illustrative embodiment of
the present invention.
[0083] FIG. 12 is an illustration of residual spectrum due to
wavelength error mismatch and a first order difference
spectrum.
[0084] FIG. 13 is an illustration of first order absorption spectra
of various compounds in a mid-IR region.
[0085] FIG. 14 is an illustration of multi-region, cross-analysis
band selection.
[0086] FIG. 15 is a block diagram of a method for multi-region,
cross-analysis band selection according to an illustrative
embodiment of the present invention.
[0087] FIG. 16 is an illustration of potential non-linear spectral
error caused by wavelength-dependent spectral magnitude
variations.
[0088] FIG. 17 is a block diagram of a method of multi-region,
cross analysis regression according to an illustrative embodiment
of the present invention.
[0089] FIG. 18 is an illustration of programmatically varying light
source intensity with dead band regions.
[0090] FIG. 19 is a block diagram for a method of modulating the
intensity of an electromagnetic radiation source according to an
illustrative embodiment of the present invention.
[0091] FIG. 20 is an illustration of synchronizing two EM radiation
sources to obtain higher modulation bandwidth, according to an
illustrative embodiment of the present invention.
[0092] FIG. 21 is an illustration of the use of two EM radiation
sources and two detectors for multi-wavelength region analysis,
according to an illustrative embodiment of the present
invention.
[0093] FIG. 22 is a process flow diagram for a spectroscopic method
using sequential, differential measurement according to an
illustrative embodiment of the present invention.
[0094] FIG. 23 is an illustration of a method for quadrupling a
clock signal from an encoder according to an illustrative
embodiment of the present invention.
[0095] FIG. 24 is an illustration of tilted filter, tilted with
respect to the rotation axis, according to an illustrative
embodiment of the present invention.
[0096] FIG. 25 is an illustration of a configuration employing
stacked filter assemblies, according to an illustrative embodiment
of the present invention.
[0097] FIG. 26 is a comparison of measurement stability between (i)
system with rotation axis perpendicular to the beam and (ii) system
with rotation axis non-perpendicular to the beam, according to an
illustrative embodiment of the present invention.
[0098] FIG. 27 is a graph illustrating the relationship of peak
transmission wavelength of a bandpass interference filter with the
incident angle.
[0099] FIG. 28 is a block diagram illustrating the spectroscopic
system in communication with a computer and its elements, according
to an illustrative embodiment of the present invention.
DETAILED DESCRIPTION
[0100] It is contemplated that methods, systems, and processes
described herein encompass variations and adaptations developed
using information from the embodiments described herein.
[0101] Throughout the description, where systems and compositions
are described as having, including, or comprising specific
components, or where processes and methods are described as having,
including, or comprising specific steps, it is contemplated that,
additionally, there are systems and compositions of the present
invention that consist essentially of, or consist of, the recited
components, and that there are processes and methods of the present
invention that consist essentially of, or consist of, the recited
processing steps.
[0102] The mention herein of any publication, for example, in the
Background section, is not an admission that the publication serves
as prior art with respect to any of the claims presented herein.
The Background section is presented for purposes of clarity and is
not meant as a description of prior art with respect to any
claim.
[0103] Headers are used herein to aid the reader and are not meant
to limit the interpretation of the subject matter described.
[0104] FIG. 1 is a block diagram of a spectroscopic system for
detecting electromagnetic (EM) radiation according to an embodiment
of the present invention. An EM radiation source 100, or multiple
EM radiation sources 100 covering one or multiple wavelength
regions, are configured to direct a beam of EM radiation to a
filter assembly 104 (as shown, having three filters). The EM
radiation source 100 may be made out of a heated filament, LED
type, or any other suitable type. The beam may be collected and
collimated by collimating optics 102, which may be made out of a
series of lenses or mirrors. The collimated beam is intercepted by
the filter assembly 104, which is shown as viewed from the top.
[0105] In certain embodiments, the filter assembly 104 is
configured for rotation. The filter assembly 104 may be positioned
relative to the collimated beam from the light source 100 such that
the axis of rotation is substantially perpendicular to the path of
the beam. Alternatively, the axis of rotation may be fixed such
that it is not perpendicular to the path of the beam, in order to
reduce or eliminate back-reflected light and/or stray light and/or
to further control the wavelength coverage.
[0106] In certain embodiments, the filter assembly 104 may have at
least three filters (106, 108, 110). Each of the filters may be a
narrow-band interference filter configured to pass a certain
narrowband of the EM radiation incident on it. These individual
filters are generally available commercially off-the-shelf. The
filter assembly 104 may be configured as indicated in FIG. 1 such
that the angle (A) between the collimated beam and a filter varies.
In addition, rotation causes the beam to be incident on the
different filters (106, 108, 110) in the assembly. The numbers of
filters that can be employed is between one and four, depending on
the measurement or monitoring application. In particular, this
depends on the number of compounds that need to be measured or
monitored. The transmission wavelength of each of the filters can
generally be tuned from its original wavelength to approximately
95% of the original wavelength assuming a maximum of 40-degree
change in the incident angle. For example, a filter that has a
nominal (at 90-degree incident angle) transmission wavelength peak
at 2000 nm can be tuned to approximately 1900 nm (0.95.times.2000
nm). If the target compounds happen to have spectral features
within this region (1900 nm-2000 nm), using only this filter is
sufficient for the measurement. Multiple filters are needed for a
wider spectral coverage. Multiple filters may also be used to
provide spectral coverage of distinct wavelength regions, i.e.
regions that are not close to each other on the wavelength scale.
For example, one filter may have a nominal transmission at 2000 nm
and the other at 8000 nm. Continuing on the beam path in FIG. 1,
the filtered beam then passes through a sample cell 112, which may
contain a sample. The sample may be gaseous, liquid or solid.
Additional optics 114, such as focusing, collimating and/or
collecting optical elements, may be used to increase the EM
radiation throughput or to better manage or direct the EM radiation
when necessary. In the sample cell 112, the beam is intercepted by
the sample, which modifies the spectrum of the original beam. The
interaction may be in the form of absorption, fluorescence or other
types of light-matter interactions. The beam may be then focused
onto an EM radiation detector 116 using focusing optics 114. The EM
radiation detector 116 may be a semiconductor based detector such
as silicon photo-diode, a pyroelectric photo-detector, or other
types of EM radiation detectors. Using an amplifier 120, a spectral
signal from the EM radiation detector 116 may be turned into an
electrical signal, and may then be converted into a digital signal
by an analog-to-digital (A/D) converter 122.
[0107] In certain embodiments, the surface of the filter(s) is
parallel to the axis of the rotation or the rotatable filter
assembly. With this configuration, the angular coverage of the
rotating filter spectrometer starts from zero incident angle. For
example, if a narrow-band filter has a nominal (zero angle) peak
transmission at 2000 nm, the starting wavelength of the spectral
coverage of the spectrometer with the surface of filter parallel to
the axis of the rotation is theoretically 2000 nm.
[0108] In certain embodiment, the surface of one, some, or all of
the filters 2400 (FIG. 24) is angularly tilted about an axis
perpendicular to the axis of rotation of the rotatable filter
assembly and the axis normal to the surface of the filter. It is
found that tilting of the filter(s) eliminates or suppresses the
back-reflected light or stray light that may cause measurement
inaccuracy, non-linearity and/or instability and results in a
significant advantage in measurement stability. FIG. 26 shows a
comparison between the measurement stability of a system having the
rotation axis perpendicular to the electromagnetic beam (2600) and
the measurement stability of a system having the rotation axis at
87 degrees to the electromagnetic beam (2601). Each plot
corresponds to a 72-hour of zero stability run, specifically, the
system was configured and calibrated for moisture analysis at
around 2.7 .mu.m, and the sample gas was dry nitrogen. Both systems
employed a least-squares regression chemometric method to predict
the moisture concentration upon a moisture calibration spectrum.
Plots 2600 and 2601 demonstrate the significant advantage in
measurement stability afforded by tilting the filter(s) in this
way.
[0109] It is also found that tilting of the filter(s) improves
control of the wavelength coverage of the spectrometer, in light of
certain filter characteristics and mechanical angular coverage of
the rotatable filter assembly. For purposes of illustration, and
without wishing to be bound by any particular theory, FIG. 27 shows
the theoretical relationship between the incident angle and the
peak transmission wavelength of a narrowband interference filter
with a nominal (zero-angle) peak wavelength of 2000 nm.
Configuration "A" illustrates a system with the surface of the
filter parallel to the axis of rotation. Assuming that the
effective angular coverage from the rotating filter assembly is
0-30 degrees, the resulting spectral coverage is approximately 2000
nm-1900 nm. On the other hand, in configuration "B", in which the
surface of the filter is tilted at 10 degrees with respect to the
rotation axis, the resulting incident angle coverage with 0-30
degrees rotation is between 10-40 degrees, resulting in a spectral
coverage of approximately 1980 nm-1830 nm, as illustrated in FIG.
27.
[0110] In some embodiments, the amplifier 120 is a fixed gain
amplifier. Alternatively, the amplifier 120 may have a variable
gain.
[0111] A digital spectral signal from the A/D converter 122 may be
fed to a processor 124 in which a real-time digital signal
processing algorithm 126 is applied. The final outcome of the
process may be quantitative chemical composition data, which may be
displayed in a display unit 128.
[0112] The use of multiple filters such as shown in FIG. 1 (as
shown, filters 106, 108, and 110) enables wide discrete spectral
coverage. For example, one filter may cover the near infrared
region around 2000 nm, and the others may cover the mid-infrared
region around 8000 nm. The EM radiation detector 116 and the EM
radiation source 100 may produce a signal at extremely different
magnitudes in the different spectral regions. For example, when
using an amplifier 120 with a fixed gain, a signal from the 8000 nm
may amount to 1 volt, whereas a signal from the 2000 nm may amount
to 1000 volts due to the higher EM radiation source output and
better detector responsivity at the near infrared region.
[0113] In certain embodiments, an amplifier 120 switches the gain
based upon an angular position of the filters as commanded by the
gain selector logic 134. In another embodiment, an amplifier 120
switches the gain based upon the magnitude of a spectral signal
itself.
[0114] The A/D convertor 122 receives its timing or clock signal
from an encoder 208 that is preferably attached rigidly to the
filter assembly 104. The encoder 208 may produce digital pulses
that correspond to an angular position of the filter assembly
104.
[0115] FIG. 2 shows an encoder 208 in more detail. The encoder 208
may include an encoder electronics unit 206 for carrying the
digital pulses corresponding to an angular position of a rotating
filter assembly 104. An example of such an off-the-shelf encoder is
EM-1-1250 made by US Digital (Vancouver, Wash.), which produces
1250 pulses per quadrature channel per rotation.
[0116] The encoder pulses may be sent to an encoder signal
processing unit 130, of which a simplified schematic of one
embodiment is shown in FIG. 3. An encoder signal processing unit
130 may include two Schmitt triggers (304, 306) that reject any
glitches or noise due to electromagnetic interference. The clean
digital pulses may then be sent to an XOR gate 308 to combine the
two quadrature signals (300, 302) from an encoder into a single
signal 310 that is doubled in frequency. For example, in some
embodiments, an encoder 208 produces 1250 quadrature pulses per
rotation. Upon exiting the encoder signal processing unit 130, the
signal 310 has a frequency at least double the frequency of one of
the quadrature signals (300, 302) to become at least 2500 pulses
per rotation. The encoder signal processing unit 130 enables
greater over-sampling and data averaging that thereby improves
wavelength stability and the system's signal-to-noise ratio.
[0117] In certain embodiments, the pulses are multiplied further in
frequency by employing a different electronics scheme. For example,
to quadruple an original clock frequency, the following scheme may
be used. As shown in FIG. 23, an edge detector 2304 may be used to
trigger a short pulse by detecting either a positive or negative
edge in the incoming signals from a first and second channel (2300,
2302). The signals from the first and second channels (2300, 2302)
are sent to the edge detector 2304 and then the same signal is
delayed to create the effect of a difference, which will be seen by
the edge detector 2304 as a reason to output a high logic. The high
state may last the duration of the delay introduced. The duration
of the pulse may be long enough to allow an A/D convertor 122 to
perform a conversion. Illustrated in FIG. 23 is an implementation
of this using logic gates. Other methods include the use of
multiple combinations of logic gates, flip-flops, logic gates and
passive components, and analog components to achieve similar
purpose. The delay can be implemented using multiple gates which
are intended to increase the pulse width out of the edge
detector.
[0118] With reference to FIG. 3, at least one component of an
encoder 208 may be rigidly attached to a filter assembly 104. This
may help to ensure that there is no mechanical compliance (or
"play") between the two elements, thus providing for a more stable
and repeatable timing or clock signal position regardless of
variations in environmental conditions, such as vibration and
temperature variations. Such an arrangement may also enable the use
of a speed reduction mechanism, such as gear or belt drives, to
optimize power transmission while maintaining repeatable and stable
clock signal positions.
[0119] The filter assembly 104 may include a table 202 for mounting
filters (as shown in FIGS. 2, 106, 108, and 110). Each filter may
be secured to the table 202 with a mounting bracket 200. The table
202 may be coupled to a shaft 204.
[0120] In certain embodiments, a speed-reduction mechanism may be
coupled to the motor that drives the filter assembly 104. A
speed-reduction mechanism may be a belt-and-pulley type, which may
provide smooth, noise-free motion. In certain embodiments, the
velocity of the rotating filter assembly 104 is adjusted and
controlled using a digital feedback control.
[0121] To further improve wavelength stability/repeatability, the
measured signal just before the A/D conversion is over-sampled,
i.e. the signal is digitized at a frequency significantly higher
than the Nyquist criterion which is required to accurately
reproduce the analog signal digitally. Such over-sampling is
achieved by employing an encoder that provides a large number of
pulses per rotation. To illustrate by example, if the to-be-measure
spectral features require a clock signal of 100 pulses per
rotation, the encoder 208 should be designed or chosen such that it
provides significantly more than 100 pulses per rotation. A
suitable encoder for this example is one that provides on the order
of 1000 pulses per rotation. The upper limit would be the maximum
allowable sampling frequency of the A/D converter 122.
[0122] This signal over-sampling may be combined with a digital
convolution step performed in the processor 124. The combination of
data over-sampling and convolution would improve the wavelength
stability or repeatability and the spectral signal-to-noise ratio.
The convolving function 402 may be a "boxcar" function, triangular
function, Gaussian function or other applicable functions. For the
purpose of wavelength stability improvement, the exact type of
convolving function 402 is less important than the width of the
function. The width of the convolving function 402 should be
maximized to the point where widening it further would alter or
broaden the actual spectral features of the measured compound. For
example, FIG. 4 shows a "box-car" convolution function 402 applied
to a raw signal 400. It can be seen that the convoluted signal 404
is shown with an improved signal-to-noise ratio without loss of any
of the relevant spectral features.
[0123] A common source of measurement instability is baseline
instabilities of the recorded spectrum, which may be due to slight
optical alignment changes (for example due to temperature
variations), light source degradation, dirty optics, etc. FIG. 5
shows a block diagram schematic of a spectral processing method
using a baseline correction algorithm to produce a processed
spectrum 502. A baseline correction algorithm 500 may be employed
to ensure long-term measurement stability. In one embodiment, a
polynomial fit is applied to the spectrum A linear or a second
order fit is generally sufficient to remove common types of
baseline variations, although a higher order fit may also be used
as long as it does not remove the relevant spectral features. In
another embodiment, a spectral differentiation is used to remove
the baseline variations. The spectral differentiation algorithm is
of the form S_new(n)=S(n+1)-S(n), or variations thereof, where
S_new is the resulting baseline-corrected spectrum, S is the
original spectrum, and n is the data element of the spectrum.
[0124] As shown in one embodiment, shown in FIG. 6, to produce the
actual measurement values, i.e. the compounds' concentration or
density values, a classical least squares analysis 600 is applied
to the processed spectrum 502. With this method, a calibration
spectra (the "K" matrix) 602, described below, is needed before
hand. This matrix contains the calibration spectra of all of the
target compounds (compounds to be measured).
[0125] In continuous monitoring applications in which the
instrument cannot be re-zeroed ("zero" or background spectrum
taken) frequently, there may be baseline variations that cannot be
completely fitted by a polynomial function. This is particularly
true with a filter-based spectroscopy system of the present
invention, which tends to be more susceptible to these type of
baseline errors due to the nonlinear wavelength-angle function. In
addition, spectral variations that are due to un-modeled
interferences such as those due to other unknown compounds may also
be present, which would also cause measurement instabilities.
[0126] In one embodiment, shown in FIG. 6, the present invention
overcomes these problems as described below. An adaptive algorithm
is designed; one that continuously and automatically modifies the
calibration spectra ("K" matrix) to account for any un-modeled
spectral variation including those associated with long-term drifts
or instabilities. FIG. 6 shows a flow chart of this algorithm
showing its basic operation. The processed spectrum (raw spectrum
upon passing through convolution and linear baseline correction) is
fitted with a calibration matrix containing the original or the
principal calibration matrix (calibration spectra of the target
compounds) 606 and a secondary calibration matrix 604. A secondary
calibration spectrum is added to the secondary calibration matrix
each time the magnitude of the residual spectrum from the CLS
analysis exceeds a certain predetermined threshold value, as
indicated by a decision step 618 in the flow chart. The rows or
columns of the secondary calibration matrix 604 are added 608 to
the rows or columns of the principal calibration matrix 606 to
obtain a modified calibration spectra or "K" matrix 602.
[0127] To illustrate by means of an example, consider a principal
calibration matrix containing three target compounds:
k.sub.A(.lamda.) for target compound A, k.sub.B(.lamda.) for target
compound B, and k.sub.C(.lamda.) for target compound C, where
k(.lamda.) is essentially a spectrum of the target compound
calibrated at a certain compound concentration or density value.
When a disturbance occurs such that the measured spectrum could not
be adequately modeled by the principal calibration matrix (as
quantified by the residual magnitude or spectrum 614, determined by
employing the step of residual computation 612), the measured
spectrum s.sub.n(.lamda.) is added to the calibration matrix. Thus,
the calibration matrix 602 becomes:
( k A ( .lamda. ) k B ( .lamda. ) k C ( .lamda. ) s n ( .lamda. ) )
Secondary calibration spectrum ##EQU00001##
where n=1, 2, 3, . . . .
[0128] There is more than one approach to compute the residual
magnitude 616 from the residual spectrum. For example, in one
embodiment, the magnitude computation of the residual spectrum
involves computing the mean value of the absolute function of the
residual spectrum. Other magnitude computation method may be used,
such as calculating the maximum value of the absolute function of
the residual spectrum.
[0129] In certain embodiments, the size of the secondary
calibration matrix 604 (the number of the secondary calibration
spectra, "n") is predetermined. In other embodiments, the size of
the secondary calibration matrix may be continuously updated or
limited based upon certain variables such as elapsed time of
measurement, orthogonality of the secondary calibration matrix to
the principal calibration matrix, and the magnitude of the residual
spectrum.
[0130] The residual magnitude threshold value used in the
comparison step "exceed threshold?" 618 may be determined by
experimentation, taking into account factors including the inherent
random spectral noise, the number of spectral averaging which
affects spectral noise, and the required stability of the
measurement.
[0131] FIG. 7 shows another embodiment of the adaptive algorithm,
in which another condition, "sufficient orthogonality" 700 is added
before a measured spectrum is added to the calibration matrix. In
this embodiment, the measured spectrum is tested whether it is
sufficiently orthogonal to each of the principal calibration
spectrum. The test involves computing the inner dot product of the
normalized vectors, s. k, where s is the normalized measured
spectrum (not shown) and k is one of the normalized principal
calibration spectra 606. The result would be between zero
(completely orthogonal) and one (completely parallel). This test is
important to ensure no spectrum that is considerably parallel to
any of the spectra of the target compounds is entered into the
calibration matrix. If that happens, the measurement results of the
target compounds would be erroneous. An orthogonality test
threshold value should be chosen to minimize this risk. In the
present embodiment, that threshold value is chosen to be 0.05.
[0132] When the orthogonality criterion is not met, the processor
124 may be configured to produce a signal ("Provide Warning" 700)
that can be used to alert the user in various ways, including
flashing an LED, generating sounds, displaying messages, etc. The
warning signal(s) tells the user that there are one or more
interference compounds that have spectral features similar to one
of the target compounds. The algorithm can also be designed such
that the warning signal provide specific messages as to which
target compound the interference compound is conflicting with.
[0133] The usefulness of the approach is demonstrated in the
following experiment. A test unit was set up to monitor N-Butane
gas as the target compound, one of the common hydrocarbons of
interest in safety monitoring application. Isopropanol vapor (IPA)
and 1,1-Difluoroethane gas were used as the interferents, both of
which are commonly used cleaning compounds. Note that
1,1-Difluoroethane (R-152a) is commonly used as the main or sole
ingredient of "dust-off" electronic cleaning products. The
high-resolution absorption spectra (1 cm.sup.-1 resolution) of the
compounds between 3200 nm and 3600 nm are shown in FIG. 8. As seen,
the spectra 800 are greatly overlapping. If a traditional
chemometric method, such as the classical least-squares or
principal component analysis technique is to be used, the spectra
of both interferents must be entered into the calibration matrix.
Otherwise, greatly erroneous readings would be produced. With an
adaptive algorithm, on the other hand, the calibration matrix needs
to contain only one spectrum, which is the spectrum of the target
gas, N-Butane.
[0134] 1,1-Difluoroethane, one of the test interference gases was
sampled by releasing it from "dust-off" product near in the inlet
of the sampling port. Similarly, IPA vapor was sampled by opening a
bottle of rubbing alcohol liquid near the sampling inlet. FIG. 9
demonstrates the ability of the system to compensate for the
interfering compounds, IPA vapor and 1,1,-Difluoroethane gas. The
top graph 900 shows butane concentration readings using an adaptive
algorithm. The maximum butane concentration error was less than 25
ppm, which also quickly disappeared (within two measurement
cycles). This happened when a large amount of IPA vapor was
introduced. Note again that the spectrum of IPA vapor was not
included in the calibration matrix. Without the adaptive algorithm,
the maximum butane concentration error would have been more than
600 ppm due to the same interference release, as can be seen on the
lower graph 902. Similarly, the interference compensation technique
worked well for the R-152 interference, in which the system
exhibited negligible error.
Wavelength Lock
[0135] Another source of measurement instability is wavelength
scale variations due to optical alignment changes, inherent
temperature dependence of the optical interference filter 3 and/or
temperature dependence of the compound's spectral features
themselves. Interference optical filters will shift to longer
wavelength with increasing temperature and shorter wavelength with
decreasing temperature. The shift is on the order of 0.01-0.2
nm/deg. Celsius. For example, a 10 deg. Celsius shift of
temperature could amount to 2 nm of wavelength scale variation,
which would degrade measurement stability. FIG. 10 illustrates a
wavelength scale error causing an apparent shift in the absorption
spectrum of the sample, potentially causing a significant
measurement error using traditional least-squares, chemometrics
approach.
[0136] A "wavelength lock" algorithm is used to compensate for the
wavelength error by wavelength shifting the measurement spectrum
prior to the least squares prediction. A block diagram depicted in
FIG. 11 shows the flow chart of the methodology. The raw spectrum
502, presumed to contain wavelength error, is modeled using a
classical least squares algorithm (CLS) 600 (explained below) or
other similar approach to obtain the spectrum residual, i.e. the
"left-over" part of the spectrum that is not fitted by the model.
CLS regression involves the following computation:
c=sK.sup.T(KK.sup.T).sup.-1
where c is a vector containing the concentration value(s), s is the
sample spectrum, and K is the calibration matrix 1106 containing
pre-determined basis spectra. The residual spectrum, r, is obtained
by performing the following computation:
r=s-c.sup.TK.sup.T
[0137] The presence of wavelength shift causes a classical
least-squares regression (CLS) residual spectrum 612 to have
similar feature characteristics with a first order difference
spectrum 1100. To illustrate by means of an example, consider the
CH.sub.4 spectra shown in FIG. 10. Using the original spectrum as
the calibration spectrum (as the K matrix) and the wavelength
shifted spectrum as the sample spectrum, s, the resulting residual
spectrum, r, is computed and shown in the top figure 1200 of FIG.
12. The first order difference spectrum of the original spectrum,
on the other hand, is shown in the bottom figure 1202 of FIG. 12.
As seen, the residual spectrum and the first order difference
spectrum exhibit great similarities, as expected. In addition, the
magnitude of the residual spectrum, computed in the step Wavelength
shift magnitude computation 1102 is proportional to the magnitude
of the wavelength shift error. These features are used to correct
the wavelength shift error as a step Wavelength correction 1104 to
obtain a wavelength corrected spectrum 1108. This wavelength error
correction algorithm may be applied continuously at a predetermined
interval, as rapid as once every scan.
[0138] Other methods may be used to correct the wavelength shift
error. One method includes monitoring the location of the peak of
the spectrum. For example, the spectral peak at .about.3315 nm in
FIG. 10 is monitored to indicate the presence of wavelength shift.
Such a method provides lower sensitivity to wavelength shift. In
addition, the presence of other spectral features or peaks (such as
those due to background or interfering compounds) may obscure the
result.
Multi-Region, Cross-Analysis Regression
[0139] To further enhance measurement sensitivity and selectivity,
and to minimize the effects of spectral non-linearities, a
multi-band, cross-analysis, least-squares regression is used. The
least-squares regression approach uses a single analysis band or
region to measure the target compound(s), i.e. using a single
calibration matrix, K, over a certain wavelength region. On the
other hand, the multi-band, cross-band regression method uses
multiple K calibration matrices for a single or multiple target
compounds. To illustrate by means of an example, we consider an
application where the spectroscopic device is used to measure the
concentrations of methane (CH.sub.4), ethane (C.sub.2H.sub.6) and
propane (C.sub.3H.sub.8) vapors in a certain process stream or in
the ambient air. FIG. 13 shows the first order absorption spectra
of the vapors in the relevant mid-infrared region. The traditional
approach uses a single region covering the whole relevant
wavelength region, for example, between 3200 and 3500 nm to perform
the analysis using a certain chemometrics algorithm such as CLS,
PLS or PCA. One embodiment makes use of multiple regions to perform
the analysis, building multiple calibration matrices and using them
simultaneously to perform the compound concentrations computation.
FIG. 14 shows possible separate band regions, regions 1, 2 and 3
for the analysis, each containing a calibration matrix "tuned" for
the analysis of one target compounds.
[0140] Furthermore, each calibration matrix (at each region)
includes the models for some of all of the other compounds present
in the sample, to account for their interferences. Following the
previous example, the calibration matrix for C.sub.3H.sub.8 (using
region 1) would contain the models to account for the spectral
features of CH.sub.4 and C.sub.2H.sub.6 located within region 1 to
minimize the interference or cross-sensitivity effects. FIG. 15
illustrates the general approach. For example, analysis of region
1, 1502, corresponding to target compound A and intereferents B, C,
and D 1500, can yield concentration values of compound A, 1506.
Similarly, analysis of region 2, 1504, yields concentration value
of compound B, 1508.
[0141] Furthermore, more than one analysis region may be used to
compute the value of a single compound, resulting in more than one
computed concentration or density values. These computed values may
later be post-processed to produce a single value or other
information. Such a method is particularly advantageous in a highly
complex sample mixtures, high-concentration samples or highly
scattering samples, where non-linear behaviors are present. To
illustrate the method by means of example, consider an absorption
spectrum shown in FIG. 16, in particular, notice how an increase in
concentration or density value affect the magnitude of the
absorption spectrum. Instead of uniform or constant magnitude
amplification across the wavelengths, the amplification itself is
wavelength dependent. As such, a single-region analysis using a
linear least-squares regression will result in a large residual
error and will not provide an accurate concentration or density
computation. The multi-region, cross-analysis regression method
solves the problem by breaking the wavelength region into smaller
pieces, each producing a computed concentration or density value.
The concentration or density values are then post-processed 1700 to
produce a single concentration or density value and/or other
pertinent information related to the state of the sample or
measurement 1702. The general approach is illustrated in FIG.
17.
Single-Beam Based Correction
[0142] A potential source of spectral baseline instability is
instrumental variations, such as light source degradation and power
variations, optics transmission degradation due to dust and
particulates, alignment changes, etc. The spectral characteristics
of most if not all of the instrumental variations can be modeled
from the spectral characteristics of the single beam spectrum
itself. The single-beam spectrum refers to the transmission
spectrum of an EM radiation source and the optical system without
the presence of a sample. The predicted spectral features of these
potential variations (the instrument-correction spectra) are
derived from the single-beam spectrum and entered into the
calibration matrix. The instrument-correction spectra may be in the
form of the pure single-beam spectrum, the derivative(s) of the
single-beam spectrum and/or other derivations of the single-beam
spectrum.
Background Calibration Method
[0143] Certain embodiments of the invention include development of
a calibration matrix, in particular, a "background" calibration
set, S.sub.background, i.e. a set containing the spectra of all of
the interfering background samples except for the target compound,
separately from the target compound's calibration set,
S.sub.target. Two separate calibration sets are produced, one that
of the background, and one that of the target compound.
[0144] The background calibration set is developed by intentionally
varying the concentration or density levels of the background
interfering compounds. The background calibration set may also
include the spectral variations model due to instrumental changes
such as light source intensity changes. Similarly, these
instrumental variations shall be simulated intentionally to build a
calibration set that completely and accurately models all of the
potential variations. Care should be taken to ensure that the
samples used to develop the background calibration set do not
include any detectable level of the target compounds. A background
calibration matrix is then developed by reducing the spectral
variations in the background calibration set into a smaller
orthogonal set of variations using PCA (principal component
analysis), PLS (partial least squares) or other similar methods. In
using PLS, the dependent variables input would be a vector of
zeroes, due to zero values of the target compound in all of the
samples.
[0145] Independently, the calibration set of the target compound is
developed by varying its levels of concentration or density within
the relevant range. Care should be taken so as not to introduce any
impurities that might have interference effects to the recorded
spectra. For example, in the case of infrared gas absorption
measurement, nitrogen or helium may be used as the balance gas in
the calibration set development as neither exhibit any infrared
absorption signals.
[0146] The background and target compound calibration sets are then
combined to produce the calibration matrix. To illustrate by means
of example, suppose 100 spectra are obtained for the background set
and 10 spectra are obtained for the target compound set. The
complete calibration set will contain 110 spectra upon which the
calibration matrix is developed.
[0147] If there is more than one target compound, the previous
steps are repeated for each additional target compound.
Interference between the target compounds shall be taken into
account by including the spectra of any other target compounds as
part of the background calibration set of the subject target
compound.
Light Source Power Modulation
[0148] To further optimize the system components' dynamic range
and/or to provide better detections of compounds having weak
signals without increasing the overall power requirement, the EM
radiation source(s) 100 may be varied in its intensity by varying
the source voltage or current programmatically, so as to provide
higher or lower intensity depending on the spectral range that is
being analyzed at each particular instant. FIG. 18 shows an example
of an EM radiation intensity variation profile. In this example,
the EM radiation source intensity is programmed such that each
filter analysis region uses a constant source intensity. In another
embodiment, the light source intensity may be varied within each or
any of the filter analysis regions. As the filter assembly 104
rotates, the path of the beam of EM radiation will become incident
on both active and inactive portions of each filter. As the beam is
incident on an inactive portion, a dead band 1800 is produced. In
an embodiment, the EM radiation source 100 is modulated in such a
way so that the power is zero or close to zero in the dead bands
1800, to minimize the average power dissipation.
[0149] In certain embodiments, the modulation command is originated
in the processor 124, which uses the angular position of the filter
assembly 104 from the encoder electronics 206 to determine the
level of source intensity to output. The output (generally in the
form of a voltage) of the processor 124 enters the EM radiation
source power electronics, which is configured to vary the voltage
(in the case of voltage mode EM radiation source) or current (in
the case of current mode EM radiation source), and thus varying the
resulting intensity of the EM radiation source 100. The processor
contains an algorithm which is used to compute the command signal
based on the angular position info from the encoder 208. A block
diagram illustrating the method is shown in FIG. 19. In another
embodiment, the EM radiation source intensity is switched between
various predetermined levels such as low, med and high, using
trigger based switch electronics, rather than a processor.
[0150] One possible limitation to this EM radiation modulation
method is the bandwidth of the EM radiation source(s). Traditional
black body sources such as those using tungsten or kanthal
filaments have low bandwidths, and thus have generally been used
for steady state application. However, some of today's black body
sources are designed with filament designs that are capable of
bandwidths up to 50 Hz or more. LED (Light Emitting Diode) and SLED
(Super Luminescence Diode) light sources are capable of higher
modulation bandwidths.
[0151] In the case of black body sources, the bandwidth capability
can be increased by employing more than one source, synchronized to
provide a series of modulations or pulses at a higher frequency or
duty cycle than obtainable with one source. FIG. 20 illustrates an
implementation of this concept where two EM radiation sources 100
are used synchronically to double the pulse frequency from what is
obtainable by a single source. In one embodiment, the beams from
the EM radiation sources 100 are combined through collecting and
collimating optical elements and directed into the rotating filter
assembly 104. In another embodiment, the filaments are packaged in
a single collecting optics unit, such as a TO-8 package, with or
without an integrated collimating optics.
Multiple Light Sources and/or Detectors Covering Multiple
Wavelength Region
[0152] More than one EM radiation source 104 and/or EM radiation
detector 116 may be used to increase the spectral coverage of the
system. Some applications require analysis in multiple wavelength
regions that are not effectively covered by a single EM radiation
source and detector. In continuous emission monitoring application,
for example, the system needs to monitor CO, CO.sub.2 and NOx in
streams containing high level of moisture. While CO and CO.sub.2
are best analyzed in the mid-infrared region, NOx is best analyzed
in the UV region due to its large interference with water vapor
spectrum. There is no single EM radiation source or detector that
can effectively cover the UV and IR regions simultaneously. A
wavelength selective device of this invention may be combined with
multiple EM radiation sources and detectors, providing a novel
integrated system. Another example requiring multiple source
arrangement is one where the EM radiation sources are LED or SLED
types where each only covers a certain narrow band region. In such
a case, multiple sources may used with a single detector
element.
[0153] In one embodiment, the EM radiation beams from the sources
100 are combined using cold/hot mirrors 2104 with a long/short
wavelength pass filter or beamsplitter on the source and detector
side. The concept is illustrated in FIG. 21. Multiple of these
"beam combiner/splitter" elements may be used for more than two
sources and/or two detectors. In another embodiment, the EM
radiation sources 100 are packaged as an integrated element,
producing a single EM radiation beam output.
Stacked Filter Assemblies
[0154] Another configuration or feature of the rotating filter
spectroscopic system is that of stacked filters or filter
assemblies, where the two or more filters or filter assemblies are
stacked along the rotation axis. FIG. 25 illustrates the concept of
the mechanical layout. One or more sources and detector may be
used. When one source or detector is used, the beam may be split or
combined using beam-splitters, beam-combiners, cold filters, hot
filters and other equivalent functioning optics. The purpose of the
stacked filters or filter assemblies may include: (i) increasing
the number of wavelength bands that can be covered with a single
motor assembly and/or (ii) covering multiple wavelength bands that
need to use two different sources and/or detectors such as the UV
and the IR region using a single motor assembly.
Differential Measurement of Process or Reaction Monitoring
[0155] A differential measurement method may be used for monitoring
certain processes such as filtration, purification, chemical and
biological reaction where comparison is made between the input and
the output streams. The concept is illustrated in FIG. 22. The
process input and output streams can be selected and sampled
sequentially at predetermined intervals using a solenoid valve
2206, as depicted in FIG. 22. A monitoring system 2204 is used to
obtain spectrum from samples taken from the input and output
streams. A spectrum obtained from one of the streams (input stream
2202 or output stream 2208) is stored as the "zero" or baseline
spectrum. The spectrum obtained from the other stream is then
referenced to the zero spectrum. In an absorption spectroscopy
measurement, the absorption spectrum A(.lamda.) is obtained by
applying the following mathematical function:
A(.lamda.)=log.sub.10{T.sub.input(.lamda.)/T.sub.output(.lamda.)}
where T.sub.input and T.sub.output are the spectra of the input
stream and the output stream respectively, and they may be
interchanged in the above equation. The above method reduces or
even eliminates potential drift or measurement instabilities due to
instrumental and/or environmental changes. Furthermore, the method
would reduce the effects of background interferences.
[0156] As illustrated in FIG. 28, the spectroscopic system 2801 may
include a computer or may otherwise share input and output with a
computer 2802 (e.g., a computer internal or external to the
spectroscopic system), the computer including software for
digitizing, receiving, storing, and or processing data
corresponding to the detected electromagnetic radiation and/or
signals created by such detected electromagnetic radiation. The
computer may also include a keyboard or other portal for user
input, and a screen for display of data to the user. The computer
may include software for process control, data acquisition, data
processing, and/or output representation. The spectroscopic system
may include a wireless system for acquisition of data and/or system
control. For example, the wireless system may allow wireless data
transfer from and/or to a computer, allowing wireless input and/or
output (and/or system control) by/to a user via a user interface
connected to the computer, such as a keyboard and/or display
screen.
Equivalents
[0157] While the invention has been particularly shown and
described with reference to specific preferred embodiments, it
should be understood by those skilled in the art that various
changes in form and detail may be made therein without departing
from the spirit and scope of the invention as defined by the
appended claims. The relevant teachings of all the references,
patents and patent applications cited herein are incorporated
herein by reference in their entirety.
* * * * *